TCP slow start and congestion avoidance problems?

Question

Having a little problem with a trace i'm examining. I know a connection is in slow start if the window size is increasing along with the amount of ACKs sent between each segment and that it will increase by the size of the ACk'd segment. However the beginning of my trace is showing numbers that do not add up (Screenshot below). What i do not know is how packet 6's window size was calculated as the maths does not add up with the previous window size and ACKs in-between. Can anyone shed any light on this?

Also i have no idea how to spot when slow start becomes congestion avoidance. Is there something that i can look out for in the trace?

Slow start seems to only go until packet 13 so should i just assume that congestion avoidance has taken over?

http://img10.imageshack.us/f/tcptrace.jpg/

Thanks for any help given! I really appreciate it

Answer 1

Your sentence starting 'I know' is incorrect, hence your confusion. You are conflating the receive window advertised by the receiver and the congestion window maintained by the sender, which does not appear in packets and which doubles on each ACK during slow start. This is not the place to reiterate all of RFC 2001 but I suggest you take another look at it.